(1) Field of the Invention
The present invention relates to color display systems, and in particular, a color display system which uses a ferroelectric liquid crystal as a polarizing surface control plate capable of effecting high speed switching.
(2) Description of Prior Art
A field sequential color display system, as color display means for a color television, has been proposed by C.B.S. Ltd., U.S.A., and the like in the 1950s. In those days, however, the color selection apparatus was of a mechanical system, and the technical resolution has not been obtained in terms of the larger type and high speed display, failing to hold the main current of the color television system. However, recently, a field sequential color display system has been reconsidered by some of fields with the development of a liquid crystal optical switch, which system is proposed in a colored display (U.S. Pat. No. 4,003,081 Specification) of a radar display which utilizes a combination of a twisted nematic type (hereinafter merely referred to a TN) liquid crystal and a colored polarizing plate.
Even so, the TN liquid crystal is slow in response time and therefore is somewhat difficult to be employed for a field sequential color television. It has been desired to use an optical shutter capable of being synchronized with higher speed frame frequencies. As such optical sutters,
i. A two frequency drive liquid crystal cell which utilizes a liquid crystal of which dielectric anisotropy is changed by a drive signal frequency is disclosed in "Fast-Switching Twisted Nematic Electro-Optical Shutter and Color Filter" (Apr. 4, 1974) written by Reins and Shanks, Electronics Letters, Vol. 10, No. P114-115 (USA). In this liquid crystal, the dielectric constant anisotropy (hereinafter abbreviated to .DELTA..epsilon.) under the electric field of low frequency signal is positive, whereas the .DELTA..epsilon. under the electric field of the high frequency signal is negative. By the combination of two drive signals of high and low frequencies, the behavior of liquid crystal molecules varies, when a high frequency signal is applied and when a low frequency signal is applied, to provide ON and OFF at high speeds. PA1 ii. The .pi. cell which utilizes an arrangement of directors wherein a reverse torque is not generated (not twisted) in an intermediate director of a liquid crystal cell by a bias signal and an adjustment of a direction of rubbing an oriented film and provides ON and OFF at high speeds caused by birefringence effect is published by Boss et al (U.S. Pat. No. 4,582,396
However, the two frequency drive liquid crystal cell of (i) requires a complicated drive circuit which can supply a high frequency signal pulses of high voltage to a capacitative load produced by a liquid crystal cell. In addition, it is difficult to obtain a uniform display over a large area. The .pi. cell of (ii) is faster in response of liquid crystal than an ordinary TN cell but a fall time is long, 2 msec, which corresponds to ten times of a rise time. When the frame frequency becomes fast, a time of an intermediate state between the rise and fall occupied during a period becomes relatively long. For this reason, color contrast and brightness lower. Moreover, since a voltage is always applied to a cell, a liquid crystal molecule has a large tilt angle even during OFF with the result that dependency on the angle of view strongly appears, and only the narrow range of color display may be observed. Because of the problems noted above, a practical field sequential full color display has been difficult to be realized.